Photolithography is the process used in the fabrication of microchips and other integrated circuits wherein light is used to transfer a geometric pattern from a mask to a photo-sensitive chemical layer, the resist, on a substrate. Following exposure, the resist is developed to yield a relief image. Optical proximity correction is a photolithography enhancement technique commonly used to compensate for image errors due to diffraction or process effects. Optical proximity correction modifies shapes in the layout pattern so the mask compensates for dimensional distortions that occur in wafer lithography processing.
At the current scales, depth of focus margins required to accurately develop the resist and etch the substrate are small enough that wafer topography variations can affect the accuracy and outcome of the photolithography process. Depth of focus or depth of field refers to the range of distance in object spaces for which object points are imaged with acceptable sharpness with a fixed position of the image plane. Wafer topography variations, due to either defects or structural design, can leave the surface of the wafer outside of the depth of focus causing the layout pattern to be out of focus when transferred to the resist.
The above-described description is merely intended to provide a contextual overview of photolithography and optical proximity correction and is not intended to be exhaustive.